Well that is ugly. Thinking of just coding it is not eeezee.
I see no ln, (natural log) Math function in the manual.
Ok, so the ugly way to do it is to make an array of pre-calculated answers.
and compare your number read against them.
It should work.. but memory might be a concern if you want to do this accurately.
/shrug.
raschemmel's statement of buying a thermal device that pumps out the right numbers might be more simple.